High purity tantalum metal and tantalum pentoxide have become increasingly important to the electronics industry in the preparation of advanced electronic materials used in the manufacture of devices such as surface acoustic wave filters, pyroelectric infrared sensors and optoelectronic devices. High purity tantalum pentoxide is also required for the preparation of tantalate x-ray phosphors for x-ray intensifier screens. The purity of tantalum metal and tantalum pentoxide used in the manufacture of such products should be greater than 99.99% and preferably approach 99.9999%.
Three major methods exist for purifying tantalum-containing materials: distillation (or chlorination), liquid-liquid solvent extraction and ion exchange.
In the distillation method, a mixture of an impure tantalum-containing material and carbon is reacted with chlorine gas at about 600.degree. to 800.degree. C. Although distillation is a well established commercial process for the purification of tantalum-containing materials, it has difficulty separating tantalum and niobium because of the small difference in the boiling points of tantalum chloride (239.degree. C.) and niobium chloride (249.degree. C.). Since niobium is a ubiquitous impurity in tantalum-containing materials, solvent extraction and ion exchange methods are preferred over distillation when high purity tantalum and tantalum pentoxide are desired.
In solvent extraction methods, an impure tantalum-containing material is dissolved in hydrofluoric acid (HF) media and brought into contact with an organic solvent. Tantalum values are selectively extracted into the organic phase, back-extracted into aqueous solution, and precipitated as tantalum hydroxide by hydrolyzing with aqueous ammonia. The precipitated hydroxide can be converted to tantalum pentoxide by calcining in air. However, multiple extraction/back-extraction cycles may be necessary to produce the desired high purity. Examples of liquid-liquid solvent extraction methods are disclosed in U.S. Pat. Nos. 3,117,833, 3,712,939 and 4,673,554.
In ion exchange methods, an impure tantalum-containing material is dissolved in HF media and then passed through an anion exchange column which selectively retains tantalum metal values. The impurities remain in solution and are discharged in the effluent. The retained tantalum values are subsequently eluted from the column and then precipitated as tantalum hydroxide as above. An example of such a method is disclosed in U.S. Pat. No. 4,446,115. However, because of the fairly small exchange capacity of the anion exchange resin, this method is expensive to apply on a commercial scale.
While the prior art focuses the purification of tantalum-containing ore and scrap, there exists a large inventory of technical grade tantalum pentoxide which could provide a relatively inexpensive raw material for the production of high purity tantalum and tantalum oxide. However, standard technical grade tantalum pentoxide contains a number of different impurities (e.g., Al, Si, F, Cl, Na, Cr, Fe, Co, Ni, Cu, Ti, Zr, Mo, Nb, and W) at levels between 500 to 10,000 parts per million (ppm). Such high impurity levels would be unacceptable for use in the above electronic and phosphor applications. Thus, it is necessary to purify technical grade tantalum pentoxide before it can be used.
Unfortunately, tantalum pentoxide is extremely difficult to dissolve even in hydrofluoric acid, especially the crystalline .beta.-Ta.sub.2 O.sub.5 phase. Direct dissolution in HF requires excessively long dissolution times and many times the amount of HF dictated by the stoichiometry of the reaction. Thus, direct dissolution of Ta.sub.2 O.sub.5 for use in conventional purification methods like solvent extraction and ion exchange is commercially impractical.
The chemical reaction for the dissolution of Ta.sub.2 O.sub.5 in HF can be written as: EQU Ta.sub.2 O.sub.5 +14HF(aq).fwdarw.2H.sub.2 TaF.sub.7 (aq)+5H.sub.2 O
The reaction kinetics of the dissolution of Ta.sub.2 O.sub.5 in HF media have been investigated by I. I. Baram, Journal of Applied Chemistry of the U.S.S.R., V. 38, 2181-88 (1965). According to the reported results, only about 5.3 g of pure Ta.sub.2 O.sub.5 (which had been calcined at 800.degree. C. for 3 hours) could be dissolved in 1 l of 14.6M HF in 4 hours at 70.degree. C. Because the dissolution reaction was reported to be first order with respect to HF concentration, it can be estimated that approximately 10.8 g of Ta.sub.2 O.sub.5 would have dissolved in 1 l of 29 M HF in 4 hours at 70.degree. C., a molar ratio of approximately 1200:1 HF/Ta.sub.2 O.sub.5. Thus, it is evident that under the above conditions, much more HF is required to dissolve Ta.sub.2 O.sub.5 than would be dictated by stoichiometry. Such a large excess of HF is not only economically undesirable but is also likely to be hazardous.
Although increasing the temperature will increase the rate of dissolution, the amount of HF required for dissolution is still many times the stoichiomteric amount. For example, over a period of 3 days at 100.degree. C., approximately 15 g of Ta.sub.2 O.sub.5 can be dissolved in 190 ml of approximately 25M HF, a molar ratio of about 140:1 HF/Ta.sub.2 O.sub.5, or about 10 times the stoichiometric amount. However, using higher dissolution temperatures also increases energy consumption and the amount of dangerous HF fumes.
Therefore, it would be a significant improvement in the art to have a method of dissolving Ta.sub.2 O.sub.5 in an HF media to produce a tantalum-containing solution which could be used directly in conventional solvent extraction and ion exchange purification methods. It would also be a significant improvement to have a method of dissolving Ta.sub.2 O.sub.5 in HF media which would require much less HF than needed for direct dissolution of Ta.sub.2 O.sub.5 in HF media under the same conditions.